This invention relates to a calcining apparatus and more particularly to a fluidized bed calcining apparatus for remotely calcining a slurry or solution feed stream which may be toxic or hazardous material such as uranium or plutonium bearing compounds.
In a typical fluidized bed calciner a feed stream or organic or inorganic material is thermally reduced to a powder which is retrieved for storage or for further chemical processing. The feed material undergoing calcination is turned into powder-like particulates by the application of heat wherein the temperature is below the melting point of the feed material yet high enough to release carbon dioxide, bound water or other volatile impurities.
In one type of fluidized bed calciner the powder-like particulates remain suspended in the calciner by the upflow of a conveying gas stream which conveys the powder through the reaction chamber of the calciner to the calciner outlet. The suspended bed of particulates which flows through the reaction chamber to the calciner outlet thereby obtains the characteristics of a fluid and thus the bed may be referred to as a fluidized bed. A perforated support member is placed below the fluidized bed but above the source of the conveying gas stream to support the bed and to direct the flow of the conveying gas. Generally, feed material is introduced into the fluidized bed calciner through a feed nozzle device which sprays the feed material into the calciner. Heat is supplied for the calcining process by a plurality of heaters internally or externally disposed to the calciner.
In general, a fluidized bed calciner may be used to reduce high purity organic or inorganic feed material to oxide powder. Thus, a fluidized bed calciner apparatus may be utilized to thermally reduce organic or inorganic feed streams containing dioxins, nitrate wastes or other hazardous materials. Consequently, a fluidized bed calciner apparatus can be employed to convert heavy metal salt slurry or solution feed streams to powder, to thermally reduce hazardous waste feed streams, to decrease waste volume and to provide intermediate chemical treatment of process streams.
In the nuclear energy industry, for example, a fluidized bed calciner apparatus can be used to calcine uranium bearing feed streams into uranium oxide powder. In this instance the feed material may include either depleted waste uranium, natural uranium or enriched uranium. In a typical nuclear industry application, the specific feed material can be either ammonium diuranate (ADU) slurry or uranyl nitrate acid solution.
There are several methods known in the art for calcining organic or inorganic material and for utilizing fluidized beds. The present invention obtains a method that combines the concept of a fluidized bed with that of a calciner to produce a fluidized bed calciner having a compact reaction chamber for use in reducing uranium or plutonium-bearing feed streams for the nuclear energy industry and for reducing other hazardous wastes in nuclear and non-nuclear applications.
One method known in the art for calcining inorganic materials is disclosed by U.S. Pat. No. 3,862,296 issued Jan. 21, 1975 in the name of J. M. Dotson et al. and entitled "Conversion Process For Waste Nitrogen-Containing Compounds" which is assigned to the General Electric Company. This patent utilizes a process for converting nitrogen-containing compounds in a fluidized bed in the presence of a reducing agent consisting of ammonia or ammonium compounds. The bed is fluidized, agitated anad pneumatically conveyed to the calciner outlet by a gaseous medium. U.S. Pat. No. 4,079,120 issued Mar. 14, 1978 in the name of E. A. Cole et al. and entitled "Uranium Dioxide Calcining Apparatus And Method" which is assigned to the Westinghouse Electric Corporation presents a continuous kiln calcining apparatus for calcining compounds of uranium such as ammonium diuranate to produce uranium dioxide. In contrast to the Dotson patent, the Cole et al. apparatus comprises a rotating kiln containing a helical screw to agitate and mechanically convey the bed to the calciner outlet.
In a fluidized bed calciner apparatus it is preferably for efficient operation that an appropriate design be employed for a bed support member. One such design is disclosed by U.S. Pat. No. 3,463,617 issued Aug. 26, 1969 in the name of S. Takeuchi and entitled "Supporting Plate For Fluidized Bed Apparatus" which is assigned to Mitsui Shipbuilding and Engineering Company Limited. This patent presents an apparatus for roasting particulate matter, the apparatus having a horizontal plate supporting a bed of the material situated above a plenum containing hot gases which flow upward through the plate to fluidize and heat the particulate matter. This support plate is designed to impart a swirling motion to hot gases flowing upward through the plate, the gases contacting the efficiency of the apparatus. The plate is comprised of hollow, rectangular blocks each having a series of ports to allow the hot gases to flow through the plate.
In addition to an appropriately designed bed support plate, it is desirable that an efficient design be utilized for the feed stream feed nozzle which is used to spary the slurry or solution feed stream into the reaction chamber and fluidized bed. An example of a feed nozzle is disclosed by U.S. Pat. No. 3,497,324 issued Feb. 24, 1970 in the name of B. F. Loewen and entitled "Dual Fluid Injector Assembly" which is assigned to the Phillips Petroleum Company. In this device which is used to introduce hydrocarbon feedstock into a carbon black furnace, a dual fluid feed nozzle assembly comprises conduits positioned in an axial, concentric relationship forming an annulus within the assembly. Feed material flows through the inner conduit while a jacket of air flows through the annulus surrounding the inner conduit. The jacket of air is used to reduce the accumulation of deposites on the discharge ends of the inner conduit and annulus.
A precursor to the present invention was the fluidized bed calciner experimental model developed by the General Electric Company Vallecitos Nuclear Center for the COPRECAL (C O P R Ecipitation and C A Lcination) research and development project sponsored by the Department of Energy. This device comprised a vertical cylindrical calciner reaction chamber. An ammonium diuranate (ADU) feed stream was sprayed into the reaction chamber through a water-cooled feed nozzle. The fluidized bed comprising shperical Inconel or similar nickel alloy beads and feed material was fluidized by an upflow of heated nitrogen gas. The Inconel beads were sprayed with feed material from the feed nozzle. The calcining process converted the feed material into a powder-like substance which adhered to the Inconel beads. The powder then separated from the surface of the beads by the agitation of the beads induced by the fluidizing gas and by the operation of a jet grinder nozzle device that utilized a continuous jet of gas to impinge the beads onto a target plate disposed in the reaction chamber. At that point, the fluidizing gas conveyed the powder to the calciner outlet. The fluidized bed temperature was to be maintained at 400.degree. C. by internal and external heaters. The calciner reaction chamber was three inches in diameter, thirty inches in length and designed to process 500 grams or uranium dioxide (UO.sub.2) per hour at a feed concentration of 325 gU/liter in 2.5M HNO.sub.3.
As part of the Wet Scrap Recycle (WSR) Development Program the Hanford Engineering Development Laboratory (HEDL), operated for the Deparatment of Energy (DOE) by the Westinghouse Electric Corporation, fabricated and tested the calciner built by General Electric. The following difficulties were encountered during opeation: (1) the efficiency of the internal heaters significantly decreased after a few hours of operation due to heater failure; (2) after shutdown, the fluidized bed could not be conveniently refluidized; (3) the feed nozzle became plugged by slurry feed material during operation and by Inconel beads during shutdown; and (4) maintaining a constant system operating temperature was difficult.
Consequently, while the prior art devices provided calciners that performed with some efficiency, these calciners evinced difficulties in maintaining a uniformly fluidized bed at high system operating temperatures.
Therefore, what is needed is a fluidized bed calciner capable of being operated at higher temperatures without heater failure, of being conveniently refluidized after shutdown, of having a means to mitigate plugging of the feed nozzle, and capable of maintaining a sufficiently constant system operating temperature.